Tai's formula Solution:

lines = int(input())
arr = []
for _ in range(lines):
    arr.append(list(map(float, input().split())))
sum = 0
for i in range(lines - 1):
    sum += (arr[i+1][0] - arr[i][0]) * (arr[i+1][1] + arr[i][1]) / 2
print(sum/1000)